Novel Metal Oxide Nanostructures for Adsorption and Photocatalytic Degradation of Organic Dyes from Aqueous Stream

Abstract

Recent research focused on the applications of nanomaterials in environmental remediation especially the treatment of natural waters, industrial and domestic waste water and the polluted underground water. Providing clean water and a clean environment for the world growing population is a challenging task. The present thesis represents an extensive view of the use of nanomaterials in environmental remediation such as water purification using single and composite metal oxide nanomaterials by sorption and photocatalysis of toxic organic dyes.
In the present study, we have synthesized 1D iron oxide nanomaterials and iron oxide based nanocomposites such as Fe2O3-SnO2, Fe2O3-CuO, Fe2O3/ZnFe2O4/ZnO, and MgFe2O4-Fe2O3 of different morphology using precipitation, hydrothermal and reflux methods. Apart from this we have also synthesized MgO nanomaterials and iron oxide impregnated mesoporous MCM-41 by wet chemical impregnation method. The obtained metal oxide nanomaterials and their nanocomposites were characterized using XRD, SEM, TEM, EDAX, XPS, Raman, FTIR, UV-Vis-DRS and BET surface area analytical techniques and were used as adsorbents and photocatalysts for decontamination of organic dyes from aqueous solutions.
We have synthesized ferrous oxalate, hematite and maghemite nanorods by precipitation method. The XRD patterns indicate the formation of different crystalline phases of ferrous oxalate (FeC2O4.2H2O), hematite (α-Fe2O3) and maghemite (γ-Fe2O3). The SEM and TEM images confirm the formation of rod shaped morphology with diameter in the range of 100-200 nm and length up to micrometers. The prepared nanorods were used as adsorbents for removal of carcinogenic Congo red dye from aqueous solution. After the batch adsorption study, the maximum adsorption capacities of the adsorbents were found to be 103, 232 and 78 mg/g for FeC2O4.2H2O, γ-Fe2O3 and α-Fe2O3 nanorods, respectively. We have also prepared Fe2O3-SnO2 composite nanorods by using same precipitation method. XRD study revealed the presence of magnetic γ-Fe2O3 phase along with SnO2 in Fe2O3–SnO2 composite. The Fe2O3–SnO2 composite nanorods were used as adsorbents for removal of Congo red dye from aqueous solution. Among different compositions, Fe2O3–SnO2 (Fe:Sn=8:2) composite nanorod showed highest percentage adsorption with sorption capacity of 182 mg/g.
Mesoporous MCM-41 and MCM-41 impregnated with iron oxide nanomaterials (Fe-MCM-41) were prepared by a facile surfactant based wet chemical method. The experimental results indicate the formation of porous nanostructure with high surface area (>800 m2/g) and particle size in the range of 200-400 nm. The mesoporous materials were used as adsorbents for the removal of Methylene blue from aqueous media. The maximum adsorption capacity of Fe-MCM-41 was found to be 194 mg/g and was higher than that of MCM-41.
MgO nanomaterials with different morphologies such as: nanorods, hierarchical nanostructures and nanoflakes were synthesized by precipitation, reflux and hydrothermal methods, respectively. The prepared nanomaterials were used as adsorbents to remove as Malachite green and Congo red from aqueous media. The hierarchical MgO nanostructure exhibited excellent adsorption performance for removal of Malachite green and Congo red with maximum sorption capacities of 1205 and 1051 mg/g, respectively. Using same synthesis methods we have used iron salt precursor along magnesium to prepare MgFe2O4 and MgFe2O4-Fe2O3 composite nanostructures. The MgFe2O4-Fe2O3 nanocomposite prepared by precipitation method was regarded as a superb photocatalyst for 99.9 % methylene blue degradation.
We have also synthesized Fe2O3-CuO composite nanorod by same precipitation method. From FESEM and TEM analysis it was observed that the spherical CuO nanoparticles are decorated uniformly onto the α-Fe2O3 nanorod surface forming a one-dimensional heteronanostructure. The obtained 1D Fe2O3-CuO nanocomposite exhibited higher photocatalytic activity than individual α-Fe2O3 nanorods and CuO nanoparticles for degradation of Methyl orange from aqueous media under solar light irradiation.
Furthermore, we have synthesized α-Fe2O3 nanoparticle and Fe2O3/ZnFe2O4, Fe2O3/ZnFe2O4/ZnO and ZnFe2O4/ZnO mixed oxide nanocomposites by varying different molar ratio of Fe and Zn using hydrothermal method. The nanocomposite with Fe:Zn=70:30 and 60:40 contains ternary Fe2O3/ZnFe2O4/ZnO phase. The nanomaterials have been used for photocatalytic degradation of Malachite green from aqueous media using solar light irradiation. The ternary Fe2O3/ZnFe2O4/ZnO (Fe:Zn=70:30) nanocomposite exhibits highest photocatalytic activity among all the prepared nanomaterials. The enhanced activity could be attributed to the cascade electron transfer from ZnFe2O4 to ZnO to Fe2O3through the interfacial potential gradient in the ternary nanostructure